After my last post, I received a few emails suggesting Lysiosquilla maculata as a possible ID.
It's difficult to assess the animal's size from the photos I put up, but the mystery mantis was much smaller than Lysiosquilla maculata tends to be.
Based on the emails and comments I received, the consensus for the mystery mantis ID is Lysiosquilla tredecimdentata.
I just realised, however, that I have a photo of a Lysiosquilla maculata from yesterday...just for you mantis shrimp lovers out there:
Correction: I received a note letting me know I mis-identified this mantis shrimp. It looks to be Lysiosquillina lisa, so the photo below is Lysiosquillina lisa in a burrow. Thank you Perezoso!